A class-G amplifier provides different voltage rails for different levels of output signals, and the class-G amplifier having rail switching mechanism can decrease power consumption and increase efficiency. Conventional rail switching mechanism may work well for heavy load (i.e. higher output current and lower output impedance), but it is not suitable for the light load (i.e. lower output current and higher output impedance). Specifically, the class-G amplifier generally has a charge-pump for adjusting the supply voltage provided to an output stage. However, power loss of the charge-pump is increased under a higher voltage rail switching frequency and a larger difference between the voltage rails, and hence the total power consumption may be worsened when the class-G amplifier operates with the light load.